This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-122928, filed Apr. 24, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a liquid crystal display element, and more particularly, to a reflective liquid crystal display element and a half-transmissive liquid crystal display element, and a method of manufacturing the same.
These days, liquid crystal display elements are applied to various fields including note-type personal computers, display monitors, car navigation systems, desk-top function calculators, medium- and small-sized TV sets, etc. Among these display elements, reflective liquid crystal display elements do not require use of a backlight, so that they can enjoy low power consumption, thinness, and lightness in weight. To make the most of these advantages, the display elements of this type are expected to be applied to displays for portable apparatuses such as mobile personal computers.
However, a conventional reflective liquid crystal display element, like paper, is designed to utilize external light for display. If it is worked in a dark place, therefore, its display screen is too dark to be viewed with ease. It cannot be used at all in total darkness, in particular.
For this reason, the efficiency of light utilization of reflective liquid crystal display elements to be developed in the future should be as high as possible. The light utilization can be improved by optimizing a polarization plate, phase difference plate, color filter, etc., adopting a guest-host mode in which neither a polarization plate nor a phase difference plate is used, or developing reflective layers with high reflection efficiency.
A method is proposed in which a cholesteric liquid crystal is used for a high-reflection reflective layer. The cholesteric reflective layer serves to reflect circularly polarized light beams in a specific direction, out of light beams incident upon it. The wavelength zone of the reflected light various depending on the helical pitch of the cholesteric liquid crystal. Accordingly, only light beams in a desired wavelength zone can be reflected if a plurality of cholesteric liquid crystal layers with different pitches are laminated to one another. If six or more layers are laminated, in particular, most of light beams in the visible light zone can be reflected. These cholesteric reflective layers, compared with conventional metallic reflective layers of aluminum or the like, absorb less light, so that their light utilization is highly efficient.
On the other hand, a half-transmissive liquid crystal display element has been developed to solve the problem of the conventional reflective liquid crystal display element that cannot be used at all in total darkness. This half-transmissive element is provided with a backlight and a half-mirror for use as a half-transmissive reflective layer such that the element can be used as a transmissive liquid crystal display element in a dark place. Further, a half-transmissive liquid crystal display element is studied such that a pinhole is formed in the reflective layer for each pixel and a micro-lens is provided for each pixel. In the case where this liquid crystal display element is used as a reflective type, compared with the conventional reflective liquid crystal display element, the brightness of its display screen is lowered only by a margin corresponding to the pinhole. In the case where the display element is used as a transmissive type, a display screen as bright as that of the transmissive liquid crystal display element can be obtained if light emitted from a backlight is converged by means of a micro-lens and transmitted through the pinhole.
If the aforesaid cholesteric reflective layer is used as the reflective layer of the half-transmissive liquid crystal display element described above, moreover, the display element can enjoy high efficiency of light utilization and can be highly visible in both light and dark places.
In a liquid crystal display element, in general, however, the polarization plate, phase difference plate, liquid crystal layer, reflective layer, etc. are formed by laminating layers that have different positive or negative refractive index anisotropies. It is believed, therefore, that the luminance and chromaticity of the display element vary depending on the visual axis, frontal or oblique, and that the viewing angle of the display element is narrow.
This is a problem essential to the reflective and half-transmissive liquid crystal display elements that are expected to utilize external light and light from the backlight with high efficiency in the aforesaid manner. In the case where the cholesteric reflective layer, which generally has unique refractive index anisotropy, is used, in particular, it is believed to exhibit a unique viewing angle characteristic.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a liquid crystal display element, capable of enjoying a display characteristic that hardly varies depending on the visual axis, frontal or oblique, and a manufacturing method therefor.
In order to achieve the above object, a liquid crystal display element according to the present invention comprises a polarization plate having a fist average refractive index for a direction perpendicular to a display plane and a second average refractive index in a direction parallel to the display plane; a phase difference plate having a first average refractive index for a direction perpendicular to the display plane and a second average refractive index for a direction parallel to the display plane; a liquid crystal layer having a first average refractive index for a direction perpendicular to the display plane and a second average refractive index for a direction parallel to the display plane; and a selectively reflective layer for reflecting part or whole of circularly polarized light in a specific direction, the selectively reflective layer having a first average refractive index for a direction perpendicular to the display plane and a second average refractive index for a direction parallel to the display plane. The polarization plate, phase difference plate, liquid crystal layer, and selectively reflective layer are formed so that the absolute value of the sum total of the product of the thickness and the difference between the first and second average refractive indexes of the polarization plate, the product of the thickness and the difference between the first and second average refractive indexes of the phase difference plate, the product of the thickness and the difference between the first and second average refractive indexes of the liquid crystal layer, and the product of the thickness and the difference between the first and second average refractive indexes of the selectively reflective layer is 50 nm or less.
According to the liquid crystal display element of the invention, the selectively reflective layer is formed of one or a plurality of layers of a cholesteric liquid crystal, and one or more layers having positive refractive index anisotropy are arranged adjacent to the reflective layer or with one or more organic layers being interposed therebetween.
According to the liquid crystal display element described above, at least one of the layers having positive refractive index anisotropy is formed of a discotic liquid crystal.
According to the liquid crystal display element described above, moreover, at least one of the layers having positive refractive index anisotropy includes a reflective layer for reflecting specifically polarized light, light in a specific wavelength zone, or specifically polarized light in a specific wavelength zone only, out of incident light.
Another liquid crystal display element according to the invention comprises one or more reflective layers capable of reflecting specifically polarized light, light in a specific wavelength zone, or specifically polarized light in a specific wavelength zone only, out of incident light, the one or more reflective layers having positive refractive index anisotropy as a whole.
A method of manufacturing a liquid crystal display element according to the invention comprises vertically aligning a discotic liquid crystal doped with a chiral agent on a transparent first insulating substrate, thereby forming one or more reflective layers having positive refractive index anisotropy; opposing a transparent second insulating substrate to the first insulating substrate; and sealing in the a liquid crystal layer between the first and second insulating substrates.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.